BuiltWith - Find out the technology behind websites
Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.
WhatRuns - Extension that helps you identify technologies used on any website at the click of a button.
Similar Tech - SimilarTech offers a Sales Insights platform which helps companies uncover their ideal market by crawling the sourcecode of over 300M sites.
TechsFound - Find Running Technologies on Website
PublicWWW - source code search engine